High-speed precision analog and mixed-signal IC applications, (such as low offset voltage operational amplifiers, analog frequency tuning circuits, voltage controlled oscillators, and analog-to-digital (ADC) and digital-to-analog (DAC) converters) require high performance CMOS and BiCMOS technologies that are ultimately limited by the accuracy of both the active and passive components of the technology. Inaccuracies and variations in thin film and thick film resistor deposition typically result in 10% to 20% variation from the designed resistor values, while the best case polycrystalline silicon-germanium (SiGe) resistance variations have been reported at 7.2%.
In addition to active and passive components, it is often necessary to implement a number of one-time programmable-read-only-memory (PROM) elements into analog and/or digital circuits. Typical applications can include; redundancy in SRAMS or other circuit building blocks and electrically programmable feature selection such as capacitance or resistance adjustments in DAC and ADC to increase bit accuracy or improve matching in RF circuit applications.
Electrical trimming of heavily doped polysilicon resistors has been well established as a reliable and manufacturable technique for producing precision resistors in integrated circuits. Although these techniques are highly accurate and easily reproducible, the electrical trim techniques require a high current electrical trim path to the resistor, which may be a severe limitation for many circuit applications. Shown in FIG. 1 is a circuit example which allows for electrical trimming of the resistor by using a diode connected to a trim pad. To trim the resistor 1, diode D1 is forward biased through VCC and the output pad, while a current density greater than 106 A/cm2 is forced through the resistor. During the trim to target value technique, the current is increased (typically as a series of current pulses with increasing amplitudes) until the resistor reaches the desired value for the circuit application. Under normal operating conditions the diodes D1 and D2 are held at VCC or Ground to ensure that the diodes are reversed biased and thus the resistor is electrically isolated from other components within the circuit. Although this method works for some circuit applications, it is of limited use for many circuit applications that cannot have diode connected trim pads or cannot tolerate the high current densities required for trimming the resistors. Because high current densities are required for trimming polysilicon resistors, additional limitations exist regarding the voltages required for trimming resistors greater than several kΩ. These limitations can be equipment related or materials related.
Electrically programmable fuses have been well established for use in integrated circuit applications. However, technology scaling is now beginning to place limitations on the use of electrically programmable fuses because of voltage limitations on gate oxides, interlayer dielectrics, and junction diffused diodes. This trend is expected to become worse as technologies continue to scale and inter-metal dielectrics migrate to low-k dielectric materials. Recent trends have tried to circumvent these limitations by adding high-voltage drain extended structures, dual gate oxide processes, or by utilizing laser trimming techniques. However each of these methods place limitations on the manufacturing of circuits. These limitations include process complexity, increased processing costs, additional die spacing requirements, and/or expensive testing equipment such as that required for laser trimming.
These is therefore a need for a method of trimming integrated circuit resistors and programming integrated circuit fuses that can be implemented on the integrated circuit without addition cost or process complexity.